my primary reason for using metal is that one has no waterproofing on them to do, as a corrosion resistant metal (in our environments) is inherently waterproof. I've been moving away from any materials that need to be waterproofed lately, and they are part of it. I am also moving to plastic/fiberglass/closed cell high density foam decks & superstructures to get away from the water damage that a single un-waterproofed spot can show after time on the bottom and heavy use. They are also considerably thinner than comparably stiff wood sub-decks.
It allowed/allows me to integrate cannon mounting cleanly into the sub deck and removes the need for me to either A: buy a wood subdeck from suppliers that is really not optimal for what I am trying to do and B: spend a long time cutting one out myself from wood with a jigsaw and not really getting what I want.
It allows very narrow stiffening ribs and cross members that would fracture if one tried to make them out of plywood. To quantify, the modulus of elasticity for aluminum is between ~6 and 12 times larger than typical plywoods (direction dependent in ply) and stainless is between 18 to 30 times larger, meaning one can greatly reduce the cross sectional area and get the same stiffness for a member. I have also found that I got a better sub-deck-deck seal with the metal than I ever did with any of the wood subdecks I have used. So I figured I'd do it again.
Density of plywood (unwaterproofed) is on the order of 600kg/m^3. it will be far higher after soaking with epoxy to waterproof (actual value then dependent on geometry of subdeck).
304 stainless: 8000 kg/m^3 (13 times that of plywood)
5052H32 aluminum: 2680 kg/m^3 (~4.5 times typical unwaterproofed plywood)
Allowable stress:
Plywood: typically below 1.6 ksi (often much much lower, very directionally dependent and part geometry dependent). Does not yield, generally
Stainless, 304: yield = 30.5 ksi, will yield up to ~58% elongation prior to fracture (19 times plywoods typical maximum stress)
Aluminum, 5052H32: yield = 28ksi, will yield up to ~12% prior to fracture (17.5 times plywoods typical maximum stress)
the stiffness at the subdeck-hull joint from pressing the sides ofthe hull in/out will vary linearly with thickness, while the stiffness of a unsuported member to a finger pressing down on it will vary with the cube of the thickness. All of the above makes a strong argument for aluminum, and a not very strong argument for stainless steel... And I note again that the numbers put in for plywood were teh best I could find, a cut up subdeck, waterproofed, will be denser, and almost certainly less stiff and lower allowable stress (the part I am least concerned with is stress I might add) than what is shown above.
see the picture below. These sections were made on my table top mill, hence the many short sections. If there are enough folks interested (and assuming it is cost effective enough, that I am not sure of) the sub deck can be cut in one piece rather than all the short sections shown below.
